Countdown to launch for Deep Impact

NASA has confirmed that its Deep Impact mission will launch tomorrow at 18:47 GMT from Florida's Cape Canaveral Space Centre. The space craft is bound for the comet Tempel 1, where it will arrive with a bang in six months time, on 4 July.

The Deep Impact mission was pulled together relatively quickly when scientists became aware that there was an opportunity to visit the comet. The craft carries a probe designed to smash into the surface of the comet at around 37,000 kilometres per hour creating a crater that could be large enough to accommodate Rome's Colosseum.

The researchers hope that as their probe, or impactor, crash lands, it will reveal more about what lies beneath the surface of the comet. Watching from a safe distance, the original space craft will record the event with the most powerful camera to fly in deep space, mission planners say.

Dr. Michael A'Hearn, Deep Impact's principal investigator, commented last year: "We know so little about the structure of cometary nuclei that we need exceptional equipment to ensure that we capture the event, whatever the details of the impact turn out to be."

The European Space Agency is also sending a probe to a comet, carrying NASA instruments. The Rosetta probe, which launched in March last year, is headed for comet Churyumov-Gerasimenko on a journey that will take it some 10 years. When it arrives, scientists plan for it to land, rather than crash, on the comet. It will carry out detailed analysis of the surface chemistry.

Both missions hope to shed light on the same question: how did the solar system form? Comets are remnants of the early days of the solar system; leftover pieces of the original matter that condensed to form the planets. Some scientists think that organic molecules needed to form life arrived thanks to comets; others suspect Earth got much of its water from collisions with these icy bodies.

Last January, NASA's Stardust mission flew within 236 kilometres of the nucleus of comet 81P/Wild 2. It flew through the comet's inner coma, the glowing cloud that surrounds the comet nucleus, taking pictures as it went. It also collected samples, which it is bringing back to Earth. It is scheduled to land in Utah in January 2006. ®